This invention relates generally to oil and gas well logging tools. More particularly, this invention relates tools for measuring the Silicon content of rock formations through the use of gamma rays generated by a pulsed neutron source.
In petroleum and hydrocarbon production, it is desirable to know the porosity and density of the subterranean formation which contains the hydrocarbon reserves. Knowledge of porosity and density is useful in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of porosity is particularly useful in older oil wells where porosity information is either insufficient or nonexistent to determine the remaining in-place oil and to determine whether sufficient oil exists to justify applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas.
If the density of the formation is known, then porosity can be determined using known equations. A variety of tools exist which allow the density of the reservoir to be determined. Most of these tools are effective in determining the density (and hence porosity) of the reservoir when the wellbore in which the tool is run is an uncased reservoir and the tool is able to contact the subterranean medium itself. However, once a well has been cased, there exists a layer of steel and concrete between the interior of the wellbore where the tool is located and the formation itself. The well casing makes it difficult for signals to pass between the tool and the reservoir and visa versa. In addition, the cement can confuse the measurement of formation properties.
U.S. patent application Ser. No. 11/115,792 of Gilchrist discloses a method and apparatus for estimating formation density of a subterranean formation using a pulsed neutron source and three gamma ray detectors. The method disclosed therein may be used in cased boreholes. The present invention is directed towards using an apparatus similar to that disclosed in Gilchrist for determining an elemental composition of formations. This problem of determining elemental composition has been addressed, for example, in U.S. Pat. No. 4,722,220 to Herron, U.S. Pat. No. 4,317,993 to Hertzog, U.S. application Ser. No. 10/916,921 (now U.S. Pat. No. 7,205,535) of Madigan et al. The method discussed here is much simpler and can be used with existing tools for density determination.